This invention relates generally to gas turbine fuels, and specifically to a method for removing alkali metal contaminants from liquid fuels.
Combustion turbines require fuels with very low contents of the alkali metals, sodium and potassium. This is because at the operating temperatures of the hot gas path components of these turbines, the alloys of which they are constructed can suffer severe corrosion damage if exposed to unacceptable levels of alkali metals contained in the combustion gases. The allowable levels of alkali metals in some conventional gas turbine liquid fuel specifications are shown in Table 1 below.
Certain advanced turbines are designed with higher temperature capability, higher strength alloys and these materials are extremely sensitive to alkali, and thus have even stricter requirements as shown in Table 2 below.
Chemical compounds of the alkali metals exist that are soluble in some gas turbine fuels. In fuels used in power generation gas turbines, however, the alkali metals are present dissolved in small amounts of water that the fuels normally contain. This water is present as suspended or separated droplets, and is not the very small amount water which is truly dissolved in the fuel. The salts of sodium and potassium are of most concern since they are the most commonly found alkali metals. The salts of lithium are of concern from a corrosion viewpoint but are not found in significant concentrations.
To counter contamination of the fuel, the gas turbine operator may do the following:
1) Allow the fuel to settle in storage tanks before use so that the fuel as burned contains little or no suspended water; or
2) Wash the fuel with high purity water and remove the water mechanically in a centrifuge or electrostatically in an electrodesalter.
These methods require accurate methods of fuel sampling and analysis to confirm that the purchased or processed fuel meets the specification for alkali metals. These methods are also extremely difficult to carry out reliably in the power plant or industrial plant environment.
This invention incorporates a prefilter and a coalescing filter in series, in the fuel supply line to the gas turbine combustor. These components remove water, and thereby alkali metals, from the fuel oil continuously, as it is delivered to the gas turbine. The prefilter removes particulates larger than about 5 to 25 xcexcm and protects the coalescing filter from contamination. The coalescing filter is made of a hydrophobic media that retards the movement of water as it passes through the filter. The water eventually coalesces in droplet form and is drained from the coalescer.
Another component of this invention is the optional incorporation of a water sensor and/or flow meter downstream of the coalescer than ensure that the coalescer is operating properly.
Accordingly, in one aspect, the present invention relates to a system for continuously removing alkali metal contaminants from liquid fuel supplied to a combustor of a gas turbine comprising a source of fuel; means for supplying the fuel to the gas turbine; a prefilter downstream of the source and upstream of the gas turbine for removing solid particulates from the liquid fuel; and a coalescer located downstream of the prefilter and upstream of the gas turbine for separating water containing alkali metals from the liquid fuel.
In another aspect, the invention relates to a method of continuously removing alkali metal contaminants from liquid fuel supplied to a gas turbine combustor comprising the steps of:
a) supplying liquid fuel from a source to a prefilter;
b) removing solid particulates from the liquid fuel in the prefilter;
c) subsequently, supplying the liquid fuel to a coalescer where water containing alkali contaminants are separated and removed; and
d) supplying liquid fuel from the coalescer to the gas turbine combustor.